This project aims to develop a novel, tissue-equivalent gel-dosimeter of ionizing radiation, which is capable of producing highly-resolved and accurate data on three dimensional dose distributions using optical tomographic densitometry. The dose-response mechanism relies on the production of Rayleigh scattering micro-particles in the gel at the site of radiation absorption, where acrylic comonomers dispersed in the gel are polymerized. Absorption and scattering of collimated light passing through the optically turbid medium produces a net attenuation of light intensity that can be measured and is directly related to the dose. Measurements of the projections of the optical density into different directions can be used to reconstruct the distribution of radiation doses within the gel. Doses of the order of 1 Gy can be measured with a spatial resolution <1mm. In Phase 1, basic optical properties of the gel have been investigated and a prototype optical scanner using a single laser beam and scanning photodiode was constructed and used successfully to create accurate 2D dose maps. The goals of Phase 2 are to reengineer the optical tomography apparatus in order to perform 3D scanning in reasonable times, to further develop the reconstruction and image analysis software of the scanner, and to further evaluate and develop formulations of polymer gel optimized for their optical properties. The latter will require studies designed to better understand the factors that affect the optical density - dose response of polymer gels. There are significant advantages to the use of this simpler optical method of reading the dose distribution stored by the gel compared to MR imaging, which is the only alternative. The polymer gel - optical scanner system will find widespread use in radiation therapy practice, for 3D measurements in homogeneous and anthropomorphic phantoms, confirmation of computerized treatment planning and in quality assurance procedures. PROPOSED COMMERCIAL APPLICATION: Acceptance testing of approximately 125 new high energy X-ray machines requires the measurement of between 6 and 12 dose distributions, using about 1,200 gels. Quality assurance procedures for the existing 2,500 linear accelerators could require 10,000 polymer-gels. Brachytherapy and experimental applications could account for 2,000 more gels. At $300 per gel, initial annual sales of gels in excess of $3 Million are anticipated. Scanner sales (25 per year at $60,000 each) would add $l.5 Million to this.